Electrochromism refers to properties that colors of a material change reversibly while electron density changes with intercalation or deintercalation of cations in an electrode structure by an electrochemical oxidation/reduction reaction occurring from changes in the applied voltage.
Transition metal oxides such as WO3, V2O5, TiO2 and NiO exhibit hybrid conduction properties capable of ion and electron conduction. In an electrolyte, when applying specific potential to an interface between a thin film electrode of these transition metal oxides and the electrolyte, atoms such as H+, Na+ or Li+ are charged or discharged. Herein, a coloring-bleaching process accompanies during the charge and discharge process, and therefore, extensive studies have been conducted as an electrode material of an electrochemical coloring device. A display device using such an electro-coloring phenomenon is capable of obtaining light transmittance at a target level by changing applied potential from the outside, and therefore, the use in electrochemical coloring display devices having a special glass type such as a curtainless window, or a mirror type is expected.
A structure of an electrochromic device is schematically illustrated in FIG. 1. More specifically, it is a structure in which a first electrode (11), a WO3 thin film (12), an electrolyte layer (3), a LiNiOx thin film (22), a second electrode (21) and a second substrate (20) are consecutively laminated on a first substrate (10).
The WO3 thin film (12) is normally formed through a sputtering process, a chemical vapor deposition (CVD) or sol-gel method. As the electrolyte layer (3), a solid or liquid state may be used.
When applying a voltage between the first electrode (11) and the second electrode (21), ions inside the electrolyte layer (3) migrate and are discolored in the WO3 thin film by reacting as in the following chemical formula.WO3 (transparent)+xe−+xM↔MxWO3 (dark blue)
Herein, cations in the electrolyte layer (3) migrate through cracks in the WO3 thin film (12) or a free volume, and in order to increase the degree of coloring, the WO3 thin film (12) needs to be thick, which causes a problem of decreasing the discoloration rate as the migration distance of the cations increases.
In addition, in forming an electrochromic device, a WO3 thin film has been prepared using a sputtering method. However, the sputtering method has a problem of being not suitable for mass production since process apparatuses are high-priced and deposition speeds are too slow to prepare a WO3 thin film having a few hundred nm thickness.